The tightly confined modal fields of single- or few-moded waveguiding lasers and amplifiers lead to a very strong interaction between any waveguided light and the active medium in the waveguiding core. Therefore, a comparatively small amount of gain medium is sufficient for providing the gain in these devices. Specifically, the gain for a given stored energy, as well as for a given absorbed pump power, is high.
However, this efficient interaction between mode and gain medium can be detrimental. Indeed, in a laser or amplifier, the achievable single-pass gain is limited to some maximum value (e.g. 50 dB). The reason is that at the maximum, a significant fraction of the pump power is converted to amplified spontaneous emission (ASE). A 10 dB higher gain results in approximately 10 dB more ASE. At these gains, the extra pump power required to increase the gain further will be prohibitively high. Since the ASE limits the gain of the device, it also limits the energy stored in the gain media. This in turn obviously limits the amount of energy that a pulse can extract from the device. Consequently, the pulse energy that can be obtained from waveguide lasers and amplifiers is limited.
Furthermore, in optically pumped lasers and amplifiers, it is essential to optimize the interaction between the gain medium and the amplified field or generated signal beam as well as the interaction between the pump beam and the gain medium. However, in typically cladding-pumped lasers and amplifiers, the interaction with the pump beam is significantly smaller than the interaction with the signal beam. Then, for a device that efficiently absorbs the pump, the interaction with the signal beam will be much stronger than what is required. Unfortunately, this excess interaction is often accompanied by excess losses for the signal beam.
In U.S. Pat. No. 6,288,835 is described a waveguide optical amplifier with improved interaction properties between the light guided along a waveguide and the rare earth dopants within an active medium by confining said active medium being the amplifying region to a ring inside a cladding around the core of said waveguide. The regions of the ring are preferably chosen where the intensity of the signal beam is substantially smaller than its peak intensity, in a cross-section of the waveguiding device perpendicular to the direction of propagation of the signal beam. Thus, the interaction between a signal light beam and an active medium are considerably reduced without necessarily changing the properties of the gain medium or reducing the confinement of the signal light.
The pumping light is supplied by a high-power laser coupled preferably at the cladding around the core of said waveguide. The luminous energy supplied to the cladding in this way will be absorbed by the core as soon as the amplified light rays propagating in the cladding cross the interface between the core and the cladding. Obviously, the efficiency of energy pumping is mostly conditioned by the efficiency of the coupling between the core and the cladding. But a problem arises with cylindrical core/cladding interfaces, namely that some light rays conveyed by the cladding will follow a helical trajectory around the core without ever impinging on the interface. The energy conveyed by these rays is therefore injected into the cladding in vain, because it is never used to amplify the signal transported by the core.
Several solutions have been proposed to solve this problem, in particular in U.S. Pat. No. 5,949,941 and WO02/03510 by producing radial protuberances at the outer surface of the cladding comprising the active medium or designing said surface following a polygonal like shape. A clear enhancement of the mode-coupling is then achieved so that the pump power is expected to be more focused on the rare-earth doped zone. Nevertheless, for some amplifiers configurations, the power conversion efficiency is still to low to be competitive and its manufacturing may be very costly.